In view of a problem that frequency resource depletion has recently been intensified in wireless communication fields, shared-frequency wireless communication is desired. FIG. 27 is a conceptual diagram showing the entirety of two wireless local area network (LAN) systems with different frequency channels, as an example of a combination of wireless communication systems sharing a frequency band.
In the figure, the wireless communication systems are provided with wireless LAN base stations 2a and 2b, and a receiver 1a. The wireless LAN base station 2a performs communications using a frequency band of CH1 having a center frequency fa. On the other hand, the wireless LAN base station 2b performs communications using a frequency band of CH5 having a center frequency fb (where fa<fb).
In this case, the receiver 1a is arranged at a position at which radio signals of both the wireless LAN base station 2a and the wireless LAN base station 2b arrive, and receives a signal in which two radio signals including a radio signal having the center frequency fa and a radio signal having the center frequency fb partially interfere with each other.
In this way, when the receiver 1a communicates with the wireless LAN base station 2a serving as its communication target, it is essential for the receiver 1a to accurately receive a desired wave even in shared-frequency wireless communication in which a transmission frequency band of the desired wave having the center frequency fa partially overlaps a transmission frequency band of an interference wave having the center frequency fb from the wireless LAN base station 2b. 
It is noted that as another example of sharing a frequency band, there is a case in which frequencies are shared between systems with different communication schemes such as combinations of a wireless LAN system, Bluetooth (registered trademark), and WiMAX (registered trademark).
For example, it is assumed that the receiver 1a shown in FIG. 27 communicates with the wireless LAN base station 2a serving as the communication target. In this case, the transmission frequency band of the desired wave having the center frequency fa from the wireless LAN base station 2a partially overlaps the transmission frequency band of the interference wave having the center frequency fb from the wireless LAN base station 2b. In such a shared-frequency wireless communication system, it is essential for the receiver 1a to accurately receive the desired wave.
In order to effectively utilize such frequencies, technology has been reported which improves the frequency utilization efficiency of all signals to be transferred in a plurality of communication systems by sharing frequency resources at the same time and at the same place using a spectrum multiplexing technique (for example, see Non-Patent Document 1).
On the other hand, Non-Patent Document 3 discloses an adaptive-modulation orthogonal frequency-division multiplexing (OFDM) system, which changes an allocated modulation scheme in accordance with a reception level of each subcarrier.